Whole blood is made up of various cellular components such as red cells, white cells and platelets suspended in its liquid component, plasma. Whole blood can be separated into its constituent components (cellular or liquid), and the desired separated component can be administered to a patient in need of that particular component. For example, platelets can be removed from the whole blood of a healthy donor, collected, and later administered to a cancer patient, whose ability to “make” platelets has been compromised by chemotherapy or radiation treatment.
Commonly, platelets are collected by introducing whole blood into a centrifuge chamber wherein the whole blood is separated into its constituent components, including platelets, based on the densities of the different components. In the separation of platelets, sometimes referred to as plateletpheresis, the platelets are often concentrated to form a layer of packed platelets with some residual plasma (hereinafter “platelet concentrate” or “PC”). Platelets may also be derived from buffy coats obtained from manually collected units of whole blood. A plurality of buffy coats are typically pooled to provide an amount or dose of platelets suitable for transfusion. The platelet product is typically stored until needed for transfusion to a patient. For storage, the platelet product is typically resuspended in a liquid medium, such as plasma and/or a synthetic storage solution.
For the stored platelets to be suitable for later administration they must substantially retain their viability and platelet function. A number of interrelated factors may affect platelet viability and function during storage. Some of these factors include the anticoagulant used for blood collection, the method used to prepare the platelets, the type of storage container used, and the medium in which the platelets are stored.
Currently, platelets may be stored for five or even seven days at 22° C. After five days, however, platelet function may become impaired. In addition to storage time, other storage conditions have been shown to affect platelet metabolism and function including pH, storage temperature, total platelet count, plasma volume, and agitation during storage.
In order to maintain viability, platelets must generate new adenosine triphosphate (ATP) continuously to meet their energy needs. As shown in FIG. 1 platelets use two metabolic pathways to generate ATP: (a) anaerobic glycolysis followed by lactic acid fermentation or (b) glycolysis followed by oxidative phosphorylation. Glycolysis results in one mole of glucose being converted to 2 moles of pyruvate, and two moles of ATP. The pyruvate can then undergo lactic acid fermentation also called anaerobic glycolysis. Although no additional ATP is produced in lactic acid fermentation, the conversion of pyruvate to lactic acid regenerates NAD+ and allows glycolysis to continue generating at least a small amount of ATP from the metabolism of glucose. Because lactic acid fermentation, which can negatively affect the pH of the medium and platelets stored therein, is stimulated by the absence of oxygen, platelets are typically stored in containers permeable to oxygen to promote oxidative phosphorylatlon and suppress lactic acid formation.
In oxidative phosphorylation, pyruvate, fatty acid or amino acids are converted to CO2 and water in the citric acid cycle. This pathway requires the presence of an adequate supply of oxygen. Glycolysis followed by oxidative phosphorylation produces 36 moles of ATP per mole of glucose and therefore is much more efficient than glycolysis followed by lactic acid fermentation.
However, rather than utilizing oxidative phosphorylation exclusively, the platelets continue to produce lactic acid through anaerobic glycolysis. Therefore, even in the presence of adequate amounts of oxygen and when stored in media containing glucose, (media such as plasma and certain synthetic storage solutions) the utilization by platelets of glycolysis coupled with lactic acid fermentation for energy production results in the concentration of lactic acid increasing over time. As noted above, the increase in lactic acid gradually acidifies the storage media. This acidification of the media alters platelet physiology and morphology such that when the pH of the media drops below about 6 the platelets may be considered nonviable. Even drops in pH that are too small to render platelets nonviable have been observed to cause decreases in the total amount of ATP. These reductions in ATP affect platelet function as ATP plays a role in for platelet adhesion and platelet aggregation. Consequently, it would be desirable to provide a storage medium for platelets that results in the prevention and/or delay of this decrease in pH.
A variety of tests have been developed which attempt to determine the quality of stored platelets and the in vivo viability of those platelets when transfused to a patient. For instance, the percentage of platelets that maintain a discoid shape (the ESC assay) and the percentage of platelets that respond appropriately to hypotonic shock (HSR assay) are two assays which are thought to correlate well with viability of stored platelets. The ESC assay measures the percentage of platelets in a sample which have discoid morphology.
The results of the HSR (Hypotonic Shock Response) assay are often considered to correlate strongly with the in vivo effectiveness of platelets when they are introduced into an individual. This assay measures the ability of platelets to recover a discoid shape after swelling in response to a hypotonic environment. Higher scores on either the HSR or ESC assay appear to correlate with increased viability of the platelets when transfused to patients. For example, an HSR assay result of about 40% or less may indicate an ineffective platelet population. The methods and uses of the HSR and ESC assays are described in more detail by Holme et al. Transfusion, January 1998; 38:31-40, which is incorporated by reference herein.
Another shape based assay is the so called “swirling assay” which has also been used as a measure of the quality of platelet concentrates. The swirling assay is based on the ability of discoid platelets to reflect light, producing a shimmering phenomenon. As described by Bertolini and Murphy, Transfusion 1994; 34:796-801 and Transfusion 1996: 36:128-132 and incorporated herein by reference, platelet samples scoring positive in a swirling assay are believed to be of higher quality than samples scoring intermediate or negative for swirling.
The presence of the glycoprotein P-selectin on the surface of platelets is also used to characterize the viability of platelets upon transfusion with the presence of P-selectin believed to indicate a loss of viability. As described by Holme et al. Transfusion 1997; 37:12-17 and incorporated herein by reference, Platelets undergo a shape change transforming from disc shaped to sphere shaped upon platelet activation. This activation is thought to involve the secretion of β-thromboglobulin from the alpha granules resulting in the appearance of P-selectin on the surface of the platelets. Antibodies directed against P-selectin, such as the monoclonal antibody CD62P, are used to detect the presence of P-selectin on the surface of platelets and have been used as a marker of platelet activation and a decreased viability of the platelets upon transfusion.
Another marker of the quality of platelets is extracellular levels of lactate dehydrogenase. Lactate dehydrogenase is an intracellular enzyme and therefore higher extracellular levels of lactate dehydrogenase are thought to reflect increased levels of platelet lysis.
A number of approaches for the storage of platelets for transfusion have been described. Although plasma is effective for storage of platelets, it may not be the ideal medium for platelet storage because plasma itself is a valuable blood component that can be used or further processed for use in the treatment of patients with other disorders. Accordingly, synthetic aqueous solutions have been developed to preserve plasma for other uses and still provide a suitable environment for stored platelets. Such solutions may be “stand alone” solutions or may be used in combination with some amount of plasma.
InterSol®, a commercially available platelet storage medium is generally described in U.S. Pat. No. 5,908,742 which is incorporated herein in its entirety. InterSol® contains sodium citrate, sodium acetate, sodium phosphate and adjusted to isoosmolarity with sodium chloride. A typical formulation of Intersol® contains 21.5 mM (3.05 g/L) dibasic sodium phosphate anhydrous (Na2HPO4), 6.7 mM (1.05 g/L) monobasic sodium phosphate (NaH2PO4.2H2O), 10.8 mM (3.18 g/L) sodium citrate 2H2O, 32.5 mM (4.42 g/L) sodium acetate 3H2O, and 77.3 mM (4.52 g/L) sodium chloride. The InterSol® solution is approximately isoosmolar (about 300 mOsm/L) with platelets and plasma, and has a pH of approximately 7.2. In certain applications (such as, but not limited to, inactivation of pathogens in platelets) InterSol® may be used in combination with plasma ratio of InterSol®/plasma ratio approximately 70%/30% to 60%/40%. Phosphate buffer in InterSol® stabilizes the pH of the solution during platelet storage.
While InterSol® has worked satisfactorily in the preservation of blood platelets, further improvements to the storage time and in vivo viability of platelets would be desirable. For example, as noted above, it would be desirable to develop a platelet storage media that reduces platelet utilizafion of lactic acid fermentation and thus slows the drop in pH that typically occurs during platelet storage. It would also be desirable to develop a synthetic storage media that requires a reduced amount of plasma (less than approximately 30%) in the synthetic storage media for the storage of platelets. It would also be desirable to provide a platelet storage media with a lower concentration of phosphate and a sufficient supply of nutrients to substantially meet the energy needs of the platelets during storage while maintaining a pH between about 6.4 and about 7.4.